EE2007
Tutorial Notes
Prepared by Pan Yan
What are we going to do with the UART?




Transmit or receive data – Continuous data stream
Control the protocol parameters – Initialization
Control the baud rate - Initialization
Handshaking – Checking & Asserting
[To
MOV
MOV
OUT
Write to a Port]
AL, data
DX, portnum
DX, AL
[To Read From a Port]
MOV DX, portnum
IN AL, DX
Registers for Serial Port (IO Port, Not CPU Registers)
Eight I/O bytes are used for each UART to access its registers.
The switch bit DLAB can be found in the line control register LCR as bit
7 at I/O address base + 3.
I/O port
Base
(3F8h)
or
(2F8h)
base + 1
base + 2
Register to port map
DLAB = 0
Read
Write
RBR☆
Receiver buffer
IER
interrupt
enable
IIR
Interrupt
identification
base + 3
base + 4
base + 5
base + 6
base + 7
LSR☆
Line status
MSR☆
Modem status
DLAB = 1
Read
Write
THR☆
Transmitter
holding
DLL (divisor latch LSB)
IER
interrupt
enable
DLM (divisor latch MSB)
IIR
Interrupt
identification
LCR☆ line control
MCR☆ modem control
factory
LSR
test
Line status
not
MSR
used
Modem status
SCR scratch
FCR
FIFO control
FCR
FIFO control
factory
test
not
used
The communication between the processor and the UART is completely
controlled by the 12 registers in the I/O address area.
RBR : Receiver buffer register (RO) – Data Buffer
THR : Transmitter holding register (WO) – Data Buffer
MSR: Modem status register (RO) – Status - Handshaking
MCR: Modem control register (R/W) – Control - Handshaking
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EE2007
Tutorial Notes
Prepared by Pan Yan
LSR: Line status register (RO) - Status – Error and Transmission
LCR : Line control register (R/W) － Control - Initiallization
Bit
0,1
2
3,4,5
6
7
Bit
0
1
2
3
4
5
6
7
LCR : line control register
Comment
Bit 1
Bit 0
Data word length
0
0
5 bits
0
1
6 bits
1
0
7 bits
1
1
8 bits
0
1 stop bit
1
1.5 stop bits (5 bits word)
2 stop bits (6,7 or 8 bits word)
Bit 5 Bit 4 Bit 3
X
x
0
No parity
0
0
1
Odd parity
0
1
1
Even parity
1
0
1
High parity (stick)
1
1
1
Low parity (stick)
0
Break signal disabled
1
Break signal enabled
0
DLAB : RBR, THR and IER accessible
1
DLAB : DLL and DLM accessible
Value
LSR : Line status register
Comment
Data available
Overrun error
Parity error
Framing error
Break signal received
THR is empty
THR empty, line idle
Erroneous data in FIFO
MSR : Modem status register
Bit
Comment
0
delta Clear to send
1
delta Data set ready
2
trailing edge Ring indicator
3
delta Carrier detect
4
Clear to send
5
Data set ready
6
Ring indicator
7
Carrier detect
MCR : Modem control register
Bit
Comment
0
Data terminal ready
1
Request to send
2
Auxiliary output 1
3
Auxiliary output 2
4
Loopback mode
5
Autoflow control
6
Reserved
7
Reserved
Refer to Sect#22.3 of AoALP for Serial Port Example Programs
Baud & Bits Per Second (BPS)
 The two most corrupted terms in telecommunications.
 Baud is the number of signal level changes per second in a line, regardless of
the information content of those signals.
 Bits per second is the rate of transfer of information bits.
 The ratio of BPS to baud depends on the information coding scheme that
you are using.
 For example, each character in asynchronous RS-232 coding includes a start
and stop bit that are not counted as information bits, so the BPS rate is
actually less than the baud rate.
 Present-day modems, on the other hand, use a set of discrete amplitude and
phase values to encode multiple bits with each signal change. This technique,
along with compression schemes and other tricks, allows BPS rates of 14,400
and higher on lines that support relatively low signal change rates (about
2400 baud for phone lines).
Devisor Setting: Divisor value = Input frequency / (baud rate * 16)
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EE2007
Tutorial Notes
Prepared by Pan Yan
Interrupt of 8086/8088
 The 8086 series of microprocessors has an Interrupt Vector
Table situated at 0000:0000 which extends for 1024 bytes.
 The Interrupt Vector table holds the address of the Interrupt
Service Routines (ISR), all four bytes in length.
 This gives us room for the 256 Interrupt Vectors.
INT (Hex)
00 - 01
02
03 - 07
IRQ
Exception Handlers
Non-Maskable IRQ
Exception Handlers
08-0F
Hardware IRQ0~7
10 - 6F
Software Interrupts
70 - 77
Hardware IRQ8~15
78 - FF
Software Interrupts
Common Uses
Non-Maskable IRQ (Parity Errors)
System Timer, Keyboard,
Serial Comms, Sound Card,
FD Ctrl, Parallel Comms.
(DOS INT, e.g. 21H)
Real Time Clock, Mouse,
HD Drive, Co-Processor
-
Go online for BIOS and DOS ISR list:
http://ee2007.cjb.net/onlinepages.html
Example Description
Int 21/AH=01h: READ CHARACTER FROM STDIN, WITH ECHO
AH = 01h
Return: AL = character read
Notes: ^C/^Break are checked, and INT 23 executed if read. …
CODE:
MOV AH, 01h
INT 21H
MOV DL, AL
Parallel Port is more straight-forward
Port Register value mapped directly to connector pins (0V or 5V)
Bit
ComRead
proc
Push
dx
D0
D1
D2
D3
D4
D5
D6
D7
Data Port
(378h)
Func. Pin.
Data0
2
Data1
3
Data2
4
Data3
5
Data4
6
Data5
7
Data6
8
Data7
9
Status Port
(379h)
Func.
Pin
Not Used
Not Used
Not Used
/ERROR
15
/SLCT
17
PE
12
/ACK
10
/BUSY
11
Control Port
(37Ah)
Func.
Pin
STROBE
1
Auto FD 14
/INIT
16
/SLCT_IN 17
IRQ7
Not Used
Not Used
Not Used
-
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EE2007
Call
Push
And
Call
WaitForChar:
Call
Test
Jz
Mov
In
Mov
Pop
Call
Mov
Pop
ret
ComRead
endp
ComWrite proc
Push
Push
Mov
Call
Push
And
Call
WaitForXmtr:
Call
Test
Jz
Mov
Mov
Out
Pop
Call
Pop
Pop
Ret
ComWrite endp
ComGetLCR proc
Push
Mov
In
Pop
Ret
ComGetLCR endp
ComSetLCR proc
Push
Mov
Out
Pop
Ret
ComSetLCR endp
Tutorial Notes
GetLCRCom
ax
al, 7fh
SetLCRCom
Prepared by Pan Yan
;Save DLAB.
;Select normal ports.
;Write LCR to turn off DLAB
GetLSRCom
;Get LSR Register
al, 00000001b
;Data Available?
WaitForChar ;Loop until data available.
dx, comport ;Read from the input port.
al, dx
dl, al
;Save character
ax
;Restore DLAB
SetLCRCom
;Write it back to LCR.
al, dl
;Restore output character.
dx
dx
ax
dl, al
GetLCRCom
ax
al, 7fh
SetLCRCom
;Save character to output
;Switch to output register.
;Save divisor latch access bit.
;Select normal i/o ports
; rather than divisor reg.
GetLSRCom
;Read LSR for xmit empty bit.
al, 00100000b
;Xmtr buffer empty?
WaitForXmtr ;Loop until empty.
al, dl
;Get output character.
dx, ComPort ;Store it in the ouput port to
dx, al
; get it on its way.
ax
;Restore divisor access bit.
SetLCRCom
ax
dx
;Return LCR value in AL.
dx
dx, comLCR
al, dx
dx
;Point at LCR register.
;Read and return LCR value.
;Write a new value to the LCR.
dx
dx, comLCR
dx, al
dx
;Point at LCR register.
;Write value in AL to the LCR.
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